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Voltage  measurement attenuator amp - del20013


LF356 opamp FET input is in a inverting amplifier configuration here. R4 + R5 make up Ri each 1/4 W MFR withstands 250V so both will take upto 500V. Use more in series for higher voltage withstand with care in PCB layout and cabinet insulation.

Rf is selected by a digital value at A-B inputs of 4052, that way R1, R2, R3, R6 are selected for four ranges giving various attenuation levels. The important thing in this circuit is the on resistance of 4052 of 100 ohms comes in series with the output resistance of opamp, so the output is taken before the cmos switch . The output of this digital attenuator should not be loaded and should be buffered before use.

Gain = Av = Rf/Ri  and Vout = -(Rf/Ri) * Vin

Voltage
                        measurement attenuator amp
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Voltage  measurement attenuator amp - del20013

Rf and Ri are 0.1% MFR, if costly, use bourns 10T trimpot or a difficult way use series parallel combination and
scratch 10% part of a network R to increase value.

(first remove paint on resistor with a blade then with ohmmeter connected scrape black film to increment value till done, seal in dust free unit). I have also made a OpAmp Calculator which you can use to calculate Values.